Device-to-Device (D2D) communication is used in wireless technologies, including ad hoc and cellular networks. Examples of D2D communication include Bluetooth and several variants of the IEEE 802.11 standards suite, such as WiFi Direct. These systems operate in an unlicensed spectrum.
Recently, D2D communications as an underlay to cellular networks have been proposed as a way to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. This is referred to as network-assisted D2D communication. Typically, it is suggested that such D2D communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
Devices that want to communicate, or even just discover each other, typically need to transmit various forms of control signaling. One example of such control signaling is the so-called discovery signal (which may possibly include a full message), which at least carries some form of identity and is transmitted by a device that wants to be discoverable by other devices. Other devices can scan for the discovery signals. Once they have detected the discovery signal, they can take the appropriate action, for example to try to initiate a connection setup with the device transmitting the discovery message.
Multiple discovery signals from different User Equipment devices (UEs) are multiplexed on the same radio resources in a combination of Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM), and possibly Code Division Multiplexing (CDM). In the Third Generation Partnership Project (3GPP), the details for the D2D discovery signals have not yet been agreed upon. However, it is likely that D2D discovery signals will be multiplexed on specific subframes (referred to as a “discovery period”) occurring at known (or signaled) positions in the radio frame (e.g., every 10 seconds). Similarly to the D2D discovery signals, it is envisioned that UEs will transmit channels for data and/or control information.
Within a discovery period, resources (time, frequency, and possibly code resources) for transmission of data and control channels (including discovery) may be assigned by a controlling node or be defined according to pre-configured patterns. In general, each channel from each UE occupies a subset of the time/frequency and possibly code resources in the system.
For certain resource allocation mechanisms (e.g., Type 2 discovery and mode 1 communication in 3GPP Long Term Evolution (LTE)), the D2D resources are assigned by the network according to predefined resource patterns. A number of resource patterns have been proposed for D2D, mainly for discovery. However, such patterns produce fragmentation of the cellular radio resources and are not suitable for multiplexing with cellular signals in a FDM fashion. Furthermore, these patterns may occur in mutual collisions even within the area controlled by the same scheduler.